Nanotechnology is a field of study related to materials on an atomic or molecular scale. Typically size ranges involved are on the scale of from approximately 1 nanometer (1 nm) to distances on the order of 400 nm or less. Nanotechnology has potential applications in a wide range of areas, including medicine, electronics, biomaterials, and energy production.
One area of potential application for nanotechnology is in connection with chemical sensors. Metal oxide semiconductors (MOSs) (such as semiconducting tin oxide) have been used as chemical sensors for a number of years and have been shown to respond to relevant chemical species such as oxygen (O2), carbon monoxide (CO), ethanol (C2H5OH), mono-nitrogen oxides (NOx), C3H6, and H2. Applications of chemical sensors include environmental monitoring, automotive applications, fire detection, and aerospace vehicles. High surface area and controlled structure are the aspects particularly relevant to sensors. Surface area is critical to gas adsorption. Correspondingly, high surface area translates into high sensitivity because the depletion layer becomes a significant fraction of the particle with decreasing particle size. Controlled structure provides the reactive sites for adsorption and their modulation of the overall conductance. Relative to micronsized grains, powders, layers, or films, nanostructures can offer 10 to 100-fold increases in each parameter. Additionally, nanostructures are more stable and less likely to sinter, thus they can yield a more stable sensor.
Moreover, nanostructures often possess unusual reactivities due to size and surface structure, reflecting defects, interstitial atoms, and incomplete bonding. Such activity can further enhance sensitivity and lower temperature operation. Operation at lower temperature can save power, and also extend operating lifetime and maintain reproducibility by preventing grain growth by sintering. Finally, lower temperature combined with structure control can advantageously yield selectivity.
Therefore, the use of nanostructures (e.g., nanotubes or nanorods) can decrease particle growth while, given the increased number of chemically sensitive particle boundaries, improving sensor sensitivity, stability, and response time. Moreover, carrier depletion (or replenishment) throughout the “bulk” nanostructure can expand the sensor dynamic range by the virtue of adsorbates leading to full charge depletion (or replenishment) with corresponding infinite or near-zero resistance, respectively. Thus, the potential advantages of nanostructures for sensor applications are clear.
However, despite the apparent advantages of nanostructures in a wide variety of applications, including sensor applications, significant challenges remain to widespread application. When creating sensor structures using nanostructures, one recognized challenge is integration of the nanostructures in a time efficient, cost effective manner. When using nanostructures as the sensing device, no matter how well the material performs as a sensor, if the ability to implement it into a sensor structure is limited, the sensing applicability will be limited as well.
Concurrent control of micro- and nanotechnology is necessary in order to achieve reliable interfaces with nanostructures. Currently nanostructures (e.g., carbon nanotubes) are often deposited onto materials primarily by adding them to a suspension, then applying the suspension in a thin film. The resulting sensor structure is random and uncontrolled, resembling straw dropped on the floor rather than a reproducibly processed material. The contacts to the sensing nanostructure are poorly defined and not reproducible.
Different approaches have been developed in an attempt to address these issues. One approach has included attempting to align nanostructures with atomic force microscopes or laser tweezers. However, this is a labor intensive approach and cannot be used for mass fabrication. Other work has involved using hydrodynamics or other methods such as traditional dielectrophoresis to align nanostructures on an existing microplatform. These techniques are performed separate from the standard microfabrication processing, and also present significant challenges to mass fabrication.
In still other cases, through random alignment, nanostructures have been buried under metallic contacts on microstructures. More recent work has involved E-Beam processing with nanodimensioned linewidth. Again, the use of these techniques are outside of standard microfabrication techniques. Other techniques, including use of the Langmuir-Blodgett method or a deposition technique known as Superlattice Nanowire Pattern Transfer (SNAP) have also been suggested to align nanostructures, but have yet to be combined with non high-resolution microfabrication techniques.
A common aspect of all previously known fabrication methods is that they do not involve standard microprocessing techniques involving over a micron in linewidth of resolution to align and form microsensors using nanostructures. Thus, the use of standard silicon processing techniques including batch processing have not been applied to nanotechnology.